The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of integrated circuit evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component or line that can be created using a fabrication process) has decreased. The ICs have been applied in various electronic devices, and an external voltage received by the electronic devices is usually different than that for operating the ICs of the electronic devices. For example, in some applications, a mobile device receives a 3.6 V power from a battery while the ICs of the mobile device operate at a voltage level ranging from 1.8 V to 0.9 V. Moreover, in some applications, an IC receives a power supply voltage and internally operating logic circuit therein at a lower voltage level in order to obtain higher operation speed and lower power consumption. In these circumstances, direct current (DC) to DC (DC-DC) regulators are usable to convert one DC voltage level to another.